A lithographic apparatus is a machine that applies a desired pattern onto a substrate, usually onto a target portion of the substrate. A lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In that instance, a patterning device, which is alternatively referred to as a “mask” or a “reticle,” may be used to generate a circuit pattern to be formed on an individual layer of the IC. This pattern can be transferred onto a target portion (e.g., comprising part of, one, or several dies) on a substrate (e.g., a silicon wafer). Transfer of the pattern is typically via imaging onto a layer of radiation-sensitive material (i.e., resist) provided on the substrate. In general, a single substrate will contain a network of adjacent target portions that are successively patterned. Known lithographic apparatus include so-called steppers, in which each target portion is irradiated by exposing an entire pattern onto the target portion at one time, and so-called “scanners,” in which each target portion is irradiated by scanning the pattern through a radiation beam in a given direction (e.g., the “scanning”-direction) while synchronously scanning the substrate parallel or anti-parallel to this direction. It is also possible to transfer the pattern from the patterning device to the substrate by imprinting the pattern onto the substrate.
In order to monitor the lithographic process, it is necessary to measure parameters of the patterned substrate such as, for example, the structure of the pattern. Knowledge of the structure of the pattern (and its accuracy) gives rise to information regarding the accuracy of the patterning system (specifically, the illumination or exposure system) that creates the pattern. The accurate formation of the pattern needs to be monitored to, for example, ensure that successive layers formed in or on the substrate are aligned to eliminate noise or cross-talk between portions of the pattern close to one another.
There are various techniques for making measurements of the microscopic structures formed in lithographic processes, including the use of scanning electron microscopes and various specialized tools. One form of specialized inspection tool is a scatterometer in which a beam of radiation is directed onto a patterned target on the surface of the substrate and the scattered or reflected beam is detected and the properties of the scattered or reflected beam are measured. By comparing the properties of the beam before and after it has been reflected or scattered by the pattern on the substrate, the properties of the pattern on the substrate can be determined. This can be done, for example, by comparing the reflected beam with data stored in a library of known measurements associated with known pattern properties.
Two main types of scatterometer are known. Spectroscopic scatterometers direct a broadband radiation beam onto the substrate and measure the spectrum (e.g., intensity as a function of wavelength) of the radiation scattered into a particular narrow angular range. Angularly-resolved scatterometers use a monochromatic radiation beam and measure the intensity of the scattered radiation as a function of angle. An extension of the use of scatterometers is the use of ellipsometers, which are systems that not only measure the intensity of the reflected light, but also measure the phase difference between different polarized states of the illuminating radiation beam that is reflected from the pattern on the surface of the substrate. Different structures (e.g., structures within the pattern) on the substrate will cause different polarized radiation to reflect (e.g., scatter/diffract) differently and properties of the structures can be determined by measuring this reflected radiation.
The scatterometer may be viewed simplistically as a system to carry out the illumination of a target and the collection of data from the reflected radiation. Such a scatterometer is often used to determine properties of the substrate or, more specifically, properties of the pattern on the substrate surface that cause the scattering/diffraction of the radiation. The properties may provide information as to the alignment of the substrate with the lithographic exposure apparatus around it. Alternatively, the properties may give information regarding the internal alignment of the substrate (namely, the alignment of its consecutively exposed layers). The properties of the pattern on the substrate may include the size, shape, and alignment of printed structures on the substrate surface or within its layers. Alignment may include overlay error (between subsequent layers). Structure parameters may include line thickness, critical dimension (CD), etc.
Sensor calibration is an essential element of known scatterometry. Sensor calibration for some known angular-resolved scatterometry systems is based upon a simplified sensor model. The simplified sensor model may be referred to as a calibration model. Imperfections in an illuminator (which provides the radiation beam for reflection and subsequent detection) and in various parts of the system that transmit signals detected by the sensor (e.g., detection of the reflected beam) are not necessarily accommodated in the model. These imperfections that are not accounted for in known sensor calibrations may be determined to be an overall calibration “residual.” As sensor models generally work using the zeroth-order reflection (i.e., diffraction), the calibration residual is a residual for zeroth-order reflection. In known systems, an approximate correction has been implemented based on this zeroth-order residual. Generally, calibration models may be created based on the assumption that the illuminator and the sensor have rotational symmetry.
In order for radiation that impinges onto the substrate to diffract (in order to be measured), a pattern with a specific shape is printed on the substrate and is often known as a scatterometry target, mark, or marker. The pattern of the scatterometry target will be referred to herein as the “pattern” or the “patterned target” (e.g., when referring to the area on the substrate surface that is being measured). The pattern may include a diffraction grating and the like, which may be an array of bars or other periodic structures. The cross-section of the pattern printed on the substrate is known as the “profile” of the pattern. The profile is typically a repeating pattern such as, for example, an array of resist lines. The individual profile of each repeated portion of pattern is the individual profile of a “unit cell” of the pattern. For metrology purposes, an average is taken of a plurality of these unit cells (i.e., an average of a repeating pattern is taken) and the average of all of the individual profiles will be referred to herein as the “profile.” What is commonly known as the profile of the pattern is hence a concatenation of a number of unit cells, which may contain local variations.
The profile is generally measured from the surface of the substrate and may include various product layers. Ideally, the pattern that is printed onto the substrate would have a predetermined shape and would be printed substantially identical each time it was printed on each layer (e.g., both in terms of the shape of the pattern and its relative position to the substrate). In practice, however, the shape, position and size of the pattern may deviate from the ideal shape on consecutive product layers because of the difficulty in creating accurate shapes at the small sizes of the patterns involved.
As mentioned above, it is possible to determine the actual shape of a pattern using cross-section scanning electron microscopes and the like. However, this involves a large amount of time, effort, and specialized apparatus and is less suited for measurements in a production environment because a separate specialized apparatus is required in line with normal apparatus in, for example, a lithographic cell.
Another way to determine the profile of a scatterometry pattern is to diffract a beam of radiation from the pattern and compare the diffraction pattern with model diffraction patterns that are stored in a library of diffraction patterns alongside the model profiles that create these model patterns.
U.S. Pub. Pat. Appl. No. 2003/0028358 to Niu et al. (hereinafter “Niu”), which is incorporate herein by reference in its entirety, describes a system in which an actual signal from a pattern is compared with a library of stored signals and the system finds a closest match of signals. The stored signals are each linked to a set of pattern profile parameters. A pattern profile parameter may, for instance, be the critical dimension (CD) or width of the pattern (which may vary with height), the height of the pattern or the angle of a side surface (or “sidewall”) of the pattern. This sidewall angle may be measured either from the surface of the substrate or from a normal to the substrate surface. Niu also describes a method to find a closest match between the measured signal and a calculated signal of a model of the scatterometry pattern, where the shape of the model depends on the values of the profile parameters in the model. In other words, various possible sets of parameter values are tested to find a set that gives rise to a signal that is as close to the actual signal that has come from the scatterometry pattern as possible. This gives a series of iterations of a “model signal.” This method is repeated iteratively until the model signal is as close as possible to the actual signal and then the model signal is stored alongside the parameters used.
In another document, U.S. Pub. Pat. Appl. No. 2004/0210402 to Opsal et al. (hereinafter “Opsal”), which is incorporated by reference herein in its entirety, defines a system that aims to reduce the number of parameters required to build up the profile of a pattern from the scatterometry signals. In doing so, the system provides “control points” around the outside of the profile shape from which the profile shape may be built up. For example, a square-profiled pattern has a single control point to show its height from the substrate surface and two points to show a width. The points are then joined up in a “dot-to-dot” fashion to give a line profile.
The systems described above only find a single profile. Further, calculations provided by the systems are required to find a “profile space” (i.e., a combination of a generic profile description with a number of profile parameters) and the possible ranges of those parameters. This combination builds up a specific profile space as required. For example, a user may choose to describe a profile by a trapezoid with parameters of width, height, and sidewall angle. The user then defines ranges for these three parameters. More complex profiles are built in a similar fashion, for example, with more complex shapes or a series of trapezoids amalgamated together.
The system described above uses a library of model profiles in order to find the best match. Other systems either do not use libraries or are used in combination with libraries. An alternative system (also described above) is an iterative method, where the parameters are given a starting value and the diffraction pattern of these starting values is calculated and compared with the measured diffraction pattern. The values of the parameters are then iteratively changed to improve the match between the iteratively modeled and the measured diffraction pattern. This iterative method may be combined with the library method.
The methods and systems described above, however, cannot be used to its full potential if there are errors in the scatterometer (particularly, if the errors in the scatterometer vary from measurement to measurement). An error may occur in the scatterometer that is used to measure the profile of the pattern. An error may also occur at any time during the scatterometry process. An error may occur during the printing of the pattern on the substrate such that there is an error in the pattern. Alternatively (or additionally), the illumination system may contain an error such that radiation that is transmitted to the patterned area of the substrate may be incorrectly aligned or have a slight error in wavelength, intensity, etc. Yet alternatively, an error may occur in the sensing of the reflected radiation, either in the optical apparatus that directs reflected radiation to the sensor, or in the sensor itself. With respect to the sensor, an anomaly may occur with the sensor itself. Dust or scratches will affect the intensity of the illumination that is detected. The impact of these anomalies may vary as a function of the angle and/or wavelength of the radiated or the reflected beam.
A scatterometer that “uses” diffraction as its metrology tool will transmit a radiation beam onto the patterned target and measure, using a sensor, the beam that is diffracted. Changing the angle and/or wavelength at which the radiation beam impinges on the pattern will cause a change in the resultant diffraction pattern. The magnitude of the change in the diffraction pattern is dependent on the properties of the pattern on the substrate. One component of the variation in the diffraction pattern is known as an “asymmetry” because there can be a difference (in intensity) between the “+1” and the “−1” diffraction orders even though both orders are a result of diffraction of the same radiation beam from the same portion of the pattern. The same can be true for higher diffraction orders. The variation arises when there is an error somewhere in the scatterometry system. The type of asymmetry detected can give clues as to where in the scatterometry system an error exists.
A scatterometer must be able to take these sorts of inconsistencies into account if it is to measure correctly the profile of a pattern.
Currently, an inspection apparatus such as an angular-resolved scatterometer is calibrated using a simplified sensor model that does not take into account all of the causes of the error. The sensor model may be calibrated using measurements on known unpatterned targets (e.g., based on zeroth-order diffraction).
In critical dimension (CD) scatterometry, one of the important performance parameters is a tool-induced error. Tool-induced error factors include rotational asymmetries in the illumination system and in sensor information transmission. The zeroth-order residual correction does not work correctly, for example, in a case of a diffraction grating as a pattern on the substrate that has a higher diffraction order.